Interpretive Summary: This report presents research data on the level of resistance to glyphosate (the active ingredient in the herbicide “RoundUp”) in horseweed and the distribution of the resistant biotype in central California. The researchers exposed leaf tissue from glyphosate-resistant and -susceptible horseweed plants to glyphosate in the laboratory and used shikimate accumulation as an indicator of target enzyme activity. The shikimate assay was also used as a non-destructive technique for determining the glyphosate sensitivity of horseweed leaf tissue collected from plants throughout thirteen Central Valley counties in a survey conducted in 2006 and 2007. The resistant horseweed biotype was nearly 5-fold more resistant to the herbicide compared to the susceptible biotype and was widely distributed throughout the region. Adoption of alternative control measures for this important weed is necessary to minimize the effects on the productivity of agriculture in the region.

Technical Abstract:
Horseweed is an increasing problem in perennial crops and non-crop areas of the Central Valley of California. Similar to the situation in glyphosate-tolerant crops in other regions, glyphosate-based weed management strategies in perennial crops and non-crop areas have resulted in selection of a glyphosate-resistant horseweed biotype in California. Research was conducted to determine the level of resistance to glyphosate in horseweed using an in vivo enzyme assay and to determine the distribution of the resistant horseweed biotype in central California. The resistant biotype was 4.8-fold more tolerant of invivo glyphosate exposure compared to the susceptible biotype, although enzyme function was inhibited in both biotypes at high glyphosate concentrations. An intermediate in vivo glyphosate dose was used to discriminate between glyphosate-resistant and -susceptible individuals in a roadside survey conducted in 2006-2007. Overall, 77.5% of the individuals tested from the Central Valley were resistant or intermediate to glyphosate. Resistant individuals were found at most locations throughout the Central Valley although the proportion of resistant individuals was slightly lower in the northern-most area. No correlation could be made between proportion of resistant or susceptible individuals and land use patterns likely due to long distance seed dispersal or different selection pressure for resistant biotypes on field margins compared to within fields. Horseweed with an economically significant level of resistance to glyphosate is already widely distributed in the Central Valley of California. Grower awareness of the problem and adoption of best management practices is needed to minimize the effects of horseweed in this highly productive and diverse agricultural region.